Liquid Crystal Display Device and Method of Driving the Same

ABSTRACT

A liquid crystal display device with low power consumption is provided by using a driver circuit and a pixel that have novel circuit structures. In a liquid crystal display device using n (n is a natural number and satisfies n≧2) bit digital video signals to display an image, n×m (m is a natural number) memory circuits and n×k (k is a natural number) non-volatile memory circuits are provided in each pixel, thereby giving the device a function of storing m frames of digital video signals in the memory circuits and a function of storing k frames of digital video signals in the non-volatile memory circuits. Once stored in the memory circuits, the digital video signals are repeatedly read out for every new frame to display a still image, so that driving of a source signal line driver circuit can be stopped during still image display. Moreover, digital video signals stored in the non-volatile memory circuits are kept stored after power supply is shut off and hence the image of the stored data can immediately be displayed next time the display device is turned on.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/675,119, filed Feb. 15, 2007, now allowed, which is a continuation ofU.S. application Ser. No. 09/930,956, filed Aug. 17, 2001, now U.S. Pat.No. 7,180,496, which claims the benefit of a foreign priorityapplication filed in Japan as Ser. No. 2000-249083 on Aug. 18, 2000, allof which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a driver circuit of a semiconductordisplay device (hereinafter referred to as display device) and a displaydevice using a driver circuit, specifically to a driver circuit of anactive matrix display device having thin film transistors that areformed on an insulator and an active matrix display device using adriver circuit. More specifically, the invention relates to a drivercircuit of an active matrix liquid crystal display device that employs adigital video signal as an image source and an active matrix liquidcrystal display device using a driver circuit.

In recent years, display devices with a semiconductor thin film formedon an insulator such as a glass substrate have gained great popularity,especially, active matrix display devices using a thin film transistor(hereinafter referred to as TFT). Any of these active matrix displaydevices using a TFT has several hundred thousands to several millionTFTs and controls electric charge of respective pixels to display animage.

A technique that is being developed lately relates to a polysilicon TFTfor simultaneously forming a pixel TFT and a driver circuit TFT. Thepixel TFT is a TFT constituting a pixel and the driver circuit TFT is aTFT constituting a driver circuit that is provided in the periphery of apixel portion. The technique is a great contribution to reduction insize and reduction in power consumption of the display devices. Owing tothe development of this technique, liquid crystal display devices arebecoming indispensable devices for, e.g., display units of mobilemachines, which lately find their application in increasingly largerfields.

FIG. 13 shows an example of a schematic diagram of a liquid crystaldisplay device driven by a digital method. A pixel portion 1308 isplaced in the center. Above the pixel portion, a source signal linedriver circuit 1301 is arranged to control source signal lines. Thesource signal line driver circuit 1301 has first latch circuits 1304,second latch circuits 1305, D/A converter circuits 1306, analog switches1307, etc. Gate signal line driver circuits 1302 for controlling gatesignal lines are arranged to the left and right of the pixel portion.Although the gate signal line driver circuits 1302 are provided on bothsides of the pixel portion in FIG. 13, only one gate signal line drivercircuit may be provided to the left or right of the pixel portion.However, it is desirable to place the gate signal line driver circuit oneach side of the pixel portion, from the viewpoint of driving efficiencyand driving reliability.

The source signal line driver circuit 1301 has a structure as the oneshown in FIG. 14. The driver circuit shown in FIG. 14 as an example is asource signal line driver circuit with a horizontal resolution of 1024pixels for 3 bit digital gray scale display. The driver circuit includesshift register circuits (SR) 1401, first latch circuits (LAT1) 1402,second latch circuits (LAT2) 1403, D/A converter circuits (or D/Aconverters: D/A) 1404, etc. Though not shown in FIG. 14, the drivercircuit may have a buffer circuit, a level shifter circuit and the likeif necessary.

Referring to FIGS. 13 and 14, the operation of the device will beexplained briefly. First, clock signals (S-CLK, S-CLKb) and start pulses(S-SP) are inputted to shift register circuits 1303 (denoted by SR inFIG. 14) and pulses are outputted sequentially. The pulses are theninputted to the first latch circuits 1304 (denoted by LAT1 in FIG. 14)so that digital video signals (digital data) also inputted to the firstlatch circuits 1304 are held therein respectively. Here, D1 is the mostsignificant bit (MSB) whereas D3 is the least significant bit (LSB).When the first latch circuits 1304 complete holding digital videosignals corresponding to one horizontal period, the digital videosignals held in the first latch circuits 1304 are transferred to thesecond latch circuits 1305 (denoted by LAT2 in FIG. 14) all at once inresponse to input of latch signals (latch pulses) during the retraceperiod.

Thereafter, the shift register circuits 1303 again operate to startholding of digital video signals corresponding to the next onehorizontal period. At the same time, the digital video signals held inthe second latch circuits 1305 are converted into analog video signalsby the D/A converter circuits 1306 (denoted by D/A in FIG. 14). Theanalog video signals converted from the digital video signals arewritten in pixels through source signal lines. An image is displayed byrepeating this operation.

In a general active matrix liquid crystal display device, screen displayis updated about sixty times for every second in order to displayanimation smoothly. In other words, it is necessary to supply digitalvideo signals for every new frame and the signals have to be written inpixels each time. Even when the image to be displayed is a still image;the same digital video signals have to be kept supplied for every newframe and a driver circuit has to process the same digital video signalsrepeatedly and continuously.

An alternative method is to write digital video signals of the stillimage in an external memory circuit once and then supply the digitalvideo signals from the external memory circuit to the liquid crystaldisplay device each time a new frame is started. However, thealternative method is no different from the above method in that theexternal memory circuit and the driver circuit of the display device arerequired continuing to operate.

Reduction in power consumption is greatly demanded particularly inmobile machines. Despite the fact that mobile machines are used mostlyin a still image mode, external circuits and driver circuits of themobile machines continue to operate even during still image display asdescribed above. Therefore, reducing power consumption is hindered.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problems, andan object of the present invention is therefore to reduce powerconsumption in an external circuit, a signal line driver circuit, andthe like while a still image is displayed by employing a novel circuit.

In order to attain the object above, the present invention uses thefollowing measures.

A plurality of memory circuits are provided in each pixel so thatdigital video signals are stored for each pixel. In the case ofdisplaying a still image, information to be written into a pixel is thesame. Therefore, once signals are written, the still image can becontinuously displayed by reading out the signals stored in the memorycircuits instead of inputting the signals each time a new frame isstarted. This means that, when a still image is to be displayed, anexternal circuit, a source signal line driver circuit, and the like canstop their operation once they finish processing signals correspondingto at least one frame.

Furthermore, some of the memory circuits arranged in each pixel arenon-volatile and digital video signals stored in the non-volatile memorycircuits once can be kept stored after power supply to the displaydevice is shut off. Accordingly, when the display device is turned onagain, there is no need to newly sample the digital video signals butthe digital signals are read out of the non-volatile memory circuits todisplay a still image. This can lead to great reduction in powerconsumption.

A liquid crystal display device according to a first aspect of thepresent invention is characterized in that:

the liquid crystal display device comprises: a plurality of pixels,wherein the plural pixels each comprises a plurality of memory circuitsand a plurality of non-volatile memory circuits.

A liquid crystal display device according to a second aspect of thepresent invention is characterized in that:

the liquid crystal display device comprises: a plurality of pixels,wherein the plural pixels each comprises:

n×m memory circuits for storing m (m is a natural number and satisfies1≦m) frames of n (n is a natural number and satisfies 2≦n) bit digitalvideo signals; and

n×k non-volatile memory circuits for storing k (k is a natural numberand is satisfies 1≦k) of the n bit digital video signals.

A liquid crystal display device according to a third aspect of thepresent invention is characterized in that:

the liquid crystal display device comprises: a plurality of pixels, theplural pixels each comprising:

a source signal line;

n (n is a natural number and satisfies 2≦n) writing gate signal lines;

n reading gate signal lines;

n writing transistors;

n reading transistors;

n×m memory circuits for storing m (m is a natural number and satisfies1≦m) frames of n bit digital video signals;

n×k non-volatile memory circuits for storing k (k is a natural numberand satisfies 1≦k) of the n bit digital video signals;

2n memory circuit selecting units;

2n non-volatile memory circuit selecting units; and

a liquid crystal element,

wherein each gate electrode of the n writing transistors is electricallyconnected to one of the n writing gate signal lines, with no two gateelectrodes sharing the same writing gate signal line,

wherein each input electrode of the n writing transistors iselectrically connected to the source signal line,

wherein each output electrode of the n writing transistors iselectrically connected to one of m circuits out of the n×m memorycircuits through one of n units out of the 2n memory circuit selectingunits, each memory circuit selecting unit is making selection for nomore than one output electrode,

wherein each output electrode of the n writing transistors iselectrically connected to one of k circuits out of the n×k non-volatilememory circuits through one of n units out of the 2n non-volatile memorycircuit selecting units, each non-volatile memory circuit selecting unitmaking selection for no more than one output electrode,

wherein each gate electrode of the n reading transistors is electricallyconnected to one of then reading gate signal lines, with no two gateelectrodes sharing the same reading gate signal line,

wherein each input electrode of the n reading transistors iselectrically connected to one of m circuits out of the n×m memorycircuits through one of n units out of the 2n memory circuit selectingunits, each memory circuit selecting unit making selection for no morethan one input electrode,

wherein each input electrode of the n reading transistors iselectrically connected to one of k circuits out of the n×k non-volatilememory circuits through one of n units of the 2n non-volatile memorycircuit selecting units, each non-volatile memory circuit selecting unitmaking selection for no more than one input electrode, and

wherein each output electrode of the n reading transistors iselectrically connected to one of electrodes of the liquid crystalelement.

A liquid crystal display device according to a fourth aspect of thepresent invention is characterized in that:

the liquid crystal display device comprises: a plurality of pixels, theplural pixels each comprising:

n (n is a natural number and satisfies 2≦n) source signal lines;

a writing gate signal line;

n reading gate signal lines;

n writing transistors;

n reading transistors;

n×m memory circuits for storing m (m is a natural number and satisfies1≦m) frames of n bit digital video signals;

n×k non-volatile memory circuits for storing k (k is a natural numberand satisfies 1≦k) of the n bit digital video signals;

2n memory circuit selecting units;

2n non-volatile memory circuit selecting units; and

a liquid crystal element,

wherein each gate electrode of the n writing transistors is electricallyconnected to the writing gate signal line,

wherein each input electrode of the n writing transistors iselectrically connected to one of the n source signal lines, with no twoinput electrodes sharing the same source signal line,

wherein each output electrode of the n writing transistors iselectrically connected to one of m circuits out of the n×m memorycircuits through one of n units out of the 2n memory circuit selectingunits, each memory circuit selecting unit making selection for no morethan one output electrode,

wherein each output electrode of the n writing transistors iselectrically connected to one of k circuits out of the n×k non-volatilememory circuits through one of n units out of the 2n non-volatile memorycircuit selecting units, each non-volatile memory circuit selecting unitmaking selection for no more than one output electrode,

wherein each gate electrode of the n reading transistors is electricallyconnected to one of then reading gate signal lines, with no two gateelectrodes sharing the same reading gate signal line,

wherein each input electrode of the n reading transistors iselectrically connected to one of m circuits out of then×m memorycircuits through one of n units out of the 2n memory circuit selectingunits, each memory circuit selecting unit making selection for no morethan one input electrode,

wherein each input electrode of the n reading transistors iselectrically connected to one of k circuits out of the n×k non-volatilememory circuits through one of n units out of the 2n non-volatile memorycircuit selecting units, each non-volatile memory circuit selecting unitmaking selection for no more than one input electrode, and

wherein each output electrode of the n reading transistors iselectrically connected to one of electrodes of the liquid crystalelement.

A liquid crystal display device according to a fifth aspect of thepresent invention is characterized in that:

wherein one of the m memory circuits is selected by one of the memorycircuit selecting units, or one of the k non-volatile memory circuits isselected by one of the non-volatile memory circuit selecting unit, tocommunicate the selected memory circuit or non-volatile memory circuitwith the output electrode of its associated writing transistor, therebywriting the digital video signals in the selected memory circuit, or

wherein one of the m memory circuits is selected by one of the memorycircuit selecting units, or one of the k non-volatile memory circuits isselected by one of the non-volatile memory circuit selecting unit, tocommunicate the selected memory circuit or non-volatile memory circuitwith the input electrode of its associated reading transistor, therebyreading the digital video signals stored in the selected memory circuit.

A liquid crystal display device according to a sixth aspect of thepresent invention further to the third aspect is characterized in that:

the display device further comprising:

shift registers for outputting sampling pulses sequentially in responseto clock signals and start pulses;

first latch circuits for holding n (n is a natural number and satisfies2≦n) bit digital video signals in response to the sampling pulses;

second latch circuits for receiving the n bit digital video signals thathave been held in the first latch circuits; and

bit selecting circuits for selecting the n bit digital video signalstransferred to the second latch circuits one bit by one bit to outputthe signals to the source signal line.

A liquid crystal display device according to a seventh aspect of thepresent invention further to the fourth aspect is characterized in that:

the display device further comprising:

shift registers for outputting sampling pulses sequentially in responseto clock signals and start pulses;

first latch circuits for holding 1 bit digital video signal out of n (nis a natural number and satisfies 2≦n) bit digital video signals inresponse to the sampling pulses; and

second latch circuits for receiving the 1 bit digital video signal thathas been held in the first latch circuits to output the 1 bit digitalvideo signal to the source signal lines.

A liquid crystal display device according to a eighth aspect of thepresent invention further to the fourth aspect is characterized in that:

the display device further comprising:

shift registers for outputting sampling pulses sequentially in responseto clock signals and start pulses;

latch circuits for holding 1 bit digital video signal in response to thesampling pulses; and

bit selecting circuits for selecting one of the source signal lines inorder to output the 1 bit digital video signal that has been held in thelatch circuits to the selected source signal line.

A liquid crystal display device according to a ninth aspect of thepresent invention is characterized in that the memory circuits arestatic random access memories (SRAM).

A liquid crystal display device according to a tenth aspect of thepresent intention is characterized in that the memory circuits areferroelectric random access memories (FeRAM).

A liquid crystal display device according to a eleventh aspect of thepresent invention is characterized in that the memory circuits aredynamic random access memories (DRAM).

A liquid crystal display device according to a twelfth aspect of thepresent invention is characterized in that the non-volatile memorycircuits are non-volatile electrically erasable programmable read onlymemories (EEPROM).

A liquid crystal display device according to a thirteenth aspect of thepresent invention is characterized in that the memory circuits areformed on a glass substrate.

A liquid crystal display device according to a fourteenth aspect of thepresent invention is characterized in that the memory circuits areformed on a plastic substrate.

A liquid crystal display device according to a fifteenth aspect of thepresent invention is characterized in that the memory circuits areformed on a stainless steel substrate.

A liquid crystal display device according to a sixteenth aspect of thepresent invention is characterized in that the memory circuits areformed on a single crystal wafer.

According to a seventeenth aspect of the present invention, a method ofdriving a liquid crystal display device using n (n is a natural numberand satisfies 2≦n) bit digital video signals to display an image, theliquid crystal display device including a source signal line drivingcircuit, a gate signal line driving circuit, and a plurality of pixelsis characterized in that:

wherein shift registers in the source signal line driving circuit outputsampling pulses, which are inputted to latch circuits, which hold thedigital video signals in response to the sampling pulses, the helddigital video signals being written in a source signal line,

wherein gate signal line selecting pulses are outputted in the gatesignal line driving circuit to select a gate signal line, and

wherein one of the following (a) through (e) is conducted in pixels inthe row of the selected gate signal line out of the plural pixels:

(a) the n bit digital video signals inputted from the source signal lineare written in memory circuits;

(b) the n bit digital video signals stored in the memory circuits areread;

(c) the n bit digital video signals inputted from the source signal lineor the n bit digital video signals stored in the memory circuits arewritten in non-volatile memory circuits;

(d) the n bit digital video signals stored in the non-volatile memorycircuits are read; and

(e) the n bit digital video signals stored in the non-volatile memorycircuits are written in the memory circuits.

According to an eighteenth aspect of the present invention, a method ofdriving a liquid crystal display device using n (n is a natural numberand satisfies 2≦n) bit digital video signals to display an image, theliquid crystal display device including a source signal line drivingcircuit, a gate signal line driving circuit, and a plurality of pixelsis characterized in that:

wherein shift registers in the source signal line driving circuit outputsampling pulses, which are inputted to latch circuits, which hold thedigital video signals in response to the sampling pulses, the helddigital video signals being written in a source signal line,

wherein gate signal line selecting pulses are outputted in the gatesignal line driving circuit to select gate signal lines in order fromthe first row, and

wherein the n bit digital video signals are written in or read out theplural pixels in order from the first row.

According to a nineteenth aspect of the present invention, a method ofdriving a liquid crystal display device using n (n is a natural numberand satisfies 2≦n) bit digital video signals to display an image, theliquid crystal display device including a source signal line drivingcircuit, a gate signal line driving circuit, and a plurality of pixelsis characterized in that:

wherein shift registers in the source signal line driving circuit outputsampling pulses, which are inputted to latch circuits, which hold thedigital video signals in response to the sampling pulses, the helddigital video signals being written in a source signal line,

wherein the gate signal line driving circuit selects a gate signal lineby outputting a gate signal line selecting pulse to an arbitrarilyspecified row of gate signal line, and

wherein the n bit digital video signals are written in or read outpixels in the arbitrarily selected row of gate signal line out of theplural pixels.

According to a twentieth aspect of the present invention, wherein,during a still image display period, the n bit digital video signalsstored in the memory circuits are repeatedly read to display a stillimage, so that the source signal line driving circuit can stop itsoperation.

According to a twenty-first aspect of the present invention, anelectronic device comprising a liquid crystal display device accordingto any one of first through sixteenth aspects.

According to a twenty-second aspect of the present invention, anelectronic device employing a method of driving a liquid crystal displaydevice according to any one of seventeenth through twentieth aspects.

According to a twenty-third aspect of the present invention, anelectronic device according to twenty-first or twenty-second aspect,wherein the electronic device is one of a television set, a personalcomputer, a portable terminal, a video camera, and a head mounteddisplay.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a circuit diagram of a pixel according to the presentinvention, the pixel having therein a plurality of memory circuits and aplurality of non-volatile memory circuits;

FIG. 2 is a diagram showing an example of the circuit structure of asource signal line driver circuit for displaying an image using a pixelof the present invention;

FIGS. 3A to 3C are timing charts for displaying an image using a pixelof the present invention;

FIG. 4 is a detailed circuit diagram of a pixel according to the presentinvention, the pixel having therein a plurality of memory circuits and aplurality of non-volatile memory circuits;

FIG. 5 is a diagram showing an example of the circuit structure of asource signal line driver circuit having no second latch circuit;

FIG. 6 is a detailed circuit diagram of a pixel to which the presentinvention is applied, the pixel being driven by the source signal linedriver circuit of FIG. 5;

FIGS. 7A to 7C are timing charts for displaying an image using thecircuits shown in FIGS. 5 and 6;

FIG. 8 is a detailed circuit diagram of a pixel of the present inventionwhen to the memory circuits are dynamic random access memories;

FIGS. 9A to 9C are diagrams exemplary showing a process of manufacturinga liquid crystal display device that has a pixel of the presentinvention;

FIGS. 10A to 10C are diagrams exemplary showing a process ofmanufacturing a liquid crystal display device that has a pixel of thepresent invention;

FIGS. 11A to 11C are diagrams exemplary showing a process ofmanufacturing a liquid crystal display device that has a pixel of thepresent invention;

FIG. 12 is a diagram exemplary showing a process of manufacturing aliquid crystal display device that has a pixel of the present invention;

FIG. 13 is a diagram showing in a simplified manner the entire circuitstructure of a conventional liquid crystal display device;

FIG. 14 is a diagram showing an example of the circuit structure of asource signal line driver circuit in a conventional liquid crystaldisplay device;

FIGS. 15A to 15F are diagrams showing examples of an electronic deviceto which a display device having a pixel of the present invention can beapplied;

FIGS. 16A to 16D are diagrams showing examples of an electronic deviceto which a display device having a pixel of the present invention can beapplied;

FIG. 17 is a diagram showing an example of the circuit structure of asource signal line driver circuit having no second latch circuit;

FIGS. 18A to 18C are timing charts for displaying an image using thecircuit shown in FIG. 17;

FIGS. 19A and 19B are diagrams showing an example of manufacturing aliquid crystal display device having a pixel of the present invention;

FIG. 20 is a diagram showing an example of a gate signal line drivercircuit using a decoder;

FIG. 21 is a block diagram of a portable information terminal employingthe present invention:

FIG. 22 is a block diagram of a cellular phone employing the presentinvention; and

FIG. 23 is a block diagram of a transmission/reception unit of thecellular phone.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment Mode

FIG. 2 shows the structure of a source signal line driver circuit andthe structure of some of pixels in a liquid crystal display device thatemploys pixels having memory circuits. The circuit is capable ofhandling 3 bit digital gray scale signals, and is composed of shiftregister circuits 201, first latch circuits 202, second latch circuits203, bit signal selecting switches 204, and pixels 205. Denoted by 210are signal lines to which signals are inputted from a gate signal linedriver circuit, or directly from the external, and descriptions of thesignal lines will be found later along with explanations of the pixels.

FIG. 1 shows a detailed circuit structure of one of the pixels 205 inFIG. 2. The pixel is for 3 bit digital gray scale, and is composed of aliquid crystal element (LC), a capacitor storage (Cs), volatile memorycircuits (A1 to A3 and B1 to B3), non-volatile memory circuits (C1 toC3), etc. Denoted by 101 is a source signal line, 102 to 104 representwriting gate signal lines, 105 to 107, reading gate signal lines, 108 to110, writing TFTs, 111 to 113, reading TFTs, and 114 to 125, memorycircuit selecting units.

The present invention is characterized in that the memory circuits ineach pixel include non-volatile memory circuits (denoted by C1 to C3 inFIG. 1) for storing at least one frame of n bit digital video signals.The rest of the memory circuits here are referred to as volatile memorycircuits on purpose to distinguish them from the non-volatile memorycircuits. However, the memory circuits A1 to A3 and B1 to B3 may notalways be volatile memory circuits but may be non-volatile ones. Thememory circuits A1 to A3 and B1 to B3 in this embodiment mode arevolatile memory circuits such as SRAM and DRAM because both writing andreading have to be done in one frame period and the writing time and thereading time need to be short enough.

FIGS. 3A to 3C show simplified operation timing in the display deviceshown in FIG. 1 in accordance with the present invention. The displaydevice is for 3 bit digital gray scales and has a VGA level resolution.A method of driving this display device will be described with referenceto FIGS. 1 to 3C. The reference symbols used in this description are thesame as those in FIGS. 1 to 3C.

Reference is made to FIG. 2 and FIGS. 3A and 3B. In FIG. 3A, frameperiods are respectively denoted by α, β, γ, and δ. The operation of thecircuit in the frame period α is described first.

Similar to the conventional driver circuit of digital driving method,clock signals (S-CLK, S-CLKb) and start pulses (S-SP) are inputted tothe shift register circuits 201 and sampling pulses are outputtedsequentially. The sampling pulses are then inputted to the first latchcircuits 202 (LAT1) so that digital video signals (digital data) alsoinputted to the first latch circuits 202 are held therein respectively.This period is referred to as dot data sampling period in thisspecification. The dot data sampling period for data corresponding toone horizontal period stretches from a to period 1 to a period 480 inFIG. 3A. The digital video signals are 3 bit signals, and D1 is the mostsignificant bit (MSB) whereas D3 is the least significant bit (LSB).When the first latch circuits 202 complete holding digital video signalscorresponding to one horizontal period, the digital video signals heldin the first latch circuits 202 are transferred to the second latchcircuits 203 (LAT2) all at once in response to input of latch signals(latch pulses) during the retrace period.

Subsequently, the first latch circuits operate to hold digital videosignals corresponding to the next horizontal period in response tosampling pulses again outputted from the shift register circuits 201.

On the other hand, the digital video signals transferred to the secondlatch circuits 203 are written in the volatile memory circuits arrangedin each pixel. As shown in FIG. 3B, the dot data sampling period of thenext column is divided into three, namely, a period I, a period II, anda period III to output the digital video signals held in the secondlatch circuits to the source signal line. At this point, the bit signalselecting switches 204 make selective connection to output the signalsof the respective bits to the source signal line in order.

In the period I, pulses are inputted to the writing gate signal line 102to turn the TFT 108 conductive and the memory circuit selecting unit 114selects the volatile memory circuit A1 so that digital video signals arewritten in the volatile memory circuit A1. Subsequently, in the periodII, pulses are inputted to the writing gate signal line 103 to turn theTFT 109 conductive and the memory circuit selecting unit 115 selects thevolatile memory circuit A2 so that digital video signals are written inthe volatile memory circuit A2. Lastly, in the period III, pulses areinputted to the writing gate signal line 104 to turn the TFT 110conductive and the memory circuit selecting unit 116 selects thevolatile memory circuit A3 so that digital video signals are written inthe volatile memory circuit A3.

The above steps complete processing of digital video signalscorresponding to one horizontal period. The periods in FIG. 3Bcorrespond to the period indicated by * in FIG. 3A. The above operationis repeated until the last stage is processed, thereby completingwriting digital video signals of one frame in the volatile memorycircuits A1 to A3.

In the display device of the present invention, 3 bit digital grayscales are obtained through a time gay scale method. Unlike the usualgray scale method in which the luminance is controlled by the level ofvoltage applied to pixels, the time gray scale method applies only twolevels of voltage to pixels to render the pixels to an ON state and anOFF state (the ON state corresponds to white and the OFF statecorresponds to black on the screen) and gray scales are createdutilizing difference in display time. When n bit gray scale display isto be obtained by the time gray scale method, the display period isdivided into n periods and the ratio of the periods is set in accordancewith power of 2, as in 2^(n−1):2^(n−2): . . . :2⁰. The length of thedisplay period of a pixel is varied depending on in which period thepixel is brought into an ON state, whereby gray scale display isattained. A pixel being in an ON state here means that a voltage isapplied between pixel electrodes whereas a pixel being in an OFF statemeans that a voltage is not applied to the pixel.

If the display period is divided into periods in accordance with otherrules than power of 2, it is still possible to obtain gray scaledisplay.

Based on the above description, the operation of the circuit in theframe period β will be explained. When writing in the volatile memorycircuits in the last stage is finished, the first frame is displayed.FIG. 3C is a diagram illustrating a 3 bit time gray scale method.Currently, digital video signals are assorted by their bits into threegroups and the groups are respectively stored in the volatile memorycircuits A1 to A3. Ts1 denotes a period of display by first bit data,Ts2 denotes a period of display by second bit data, and Ts3 denotes aperiod of display by third bit data. The lengths of these displayperiods are set to satisfy Ts1:Ts2:Ts3=4:2:1.

Since the signals here are 3 bit signals, the luminance obtained rangesfrom 0 to 7, namely, 8 levels in total. When a pixel is not brought intoan ON state in any of the periods Ts1 to Ts3, the luminance is 0. When apixel is in an ON state throughout the entire periods, the luminance is7. If a pixel is to have the fifth level of luminance, the pixel isbrought into ON state in Ts1 and Ts3.

Specific explanations will be given referring to the drawings. In theperiod Ts1, pulses are inputted to the reading gate signal line 105 toturn the ITT 111 conductive and the memory circuit selecting unit 117selects the volatile memory circuit A1 so that the pixel is driven inaccordance with the digital video signals stored in the volatile memorycircuit A1. Subsequently, in Ts2, pulses are inputted to the readinggate signal line 106 to turn the TFT 112 conductive and the memorycircuit selecting unit 118 selects the volatile memory circuit A2 sothat the pixel is driven in accordance with the digital video signalsstored in the volatile memory circuit A2. Lastly, in Ts3, pulses areinputted to the reading gate signal line 107 to turn the TFT 113conductive and the memory circuit selecting unit 119 selects thevolatile memory circuit A3 so that a voltage is applied to the pixel inaccordance with the digital video signals stored in the volatile memorycircuit A3.

When the display device is a liquid crystal display device, there are anormally white mode and a normally black mode. In both modes, white andblack are inverted when a pixel switches between an ON state and an OFFstate. Accordingly, the luminance may be reverse to what is described inthe above.

Data corresponding to one frame period are displayed as above.Meanwhile, the driver circuit is processing digital video signals of thenext frame period. The procedure is the same as the one described aboveup through transferring the digital video signals to the second latchcircuits. In the subsequent volatile memory circuit writing period, theother volatile memory circuits are used However, if the number ofvolatile memory circuits in the pixel is not large enough for more thanone frame, data of the next frame are overwritten in the volatile memorycircuits in which data of the previous frame have already been written.

In the period I, pulses are inputted to the writing gate signal line 102to turn the TFT 108 conductive and the memory circuit selecting unit 114selects the volatile memory circuit B1 so that digital video signals arewritten in the volatile memory circuit B1. Subsequently, in the periodII, pulses are inputted to the writing gate signal line 103 to turn theTFT 109 conductive and the memory circuit selecting unit 115 selects thevolatile memory circuit B2 so that digital video signals are written inthe volatile memory circuit B2. Lastly, in the period III, pulses areinputted to the writing gate signal line 104 to turn the TFT 110conductive and the memory circuit selecting unit 116 selects thevolatile memory circuit B3 so that digital video signals are written inthe volatile memory circuit B3.

Then the frame period γ is started and the second frame is displayed inaccordance with the digital video signals stored in the volatile memorycircuits B1 to B3. At the same time, processing of digital video signalsof the next frame period is commenced. The digital video signals of thenext frame period are stored again in the volatile memory circuits A1 toA3 that have finished their operations related to display of the firstframe period.

Thereafter, the digital video signals stored in the volatile memorycircuits A1 to A3 are displayed in the frame period δ, and digital videosignals of the next frame period simultaneously begin to receiveprocessing. The digital video signals the next frame period are storedagain in the volatile memory circuits B1 to B3 that have finished theiroperations related to display of the second frame period.

Writing digital video signals in the non-volatile memory circuits C1 toC3 usually takes much longer time than writing digital video signals involatile memory circuits such as SRAM. Therefore, desirable procedure isto store digital video signals in the volatile memory circuits A1 to A3or B1 to B3 and then write the stored signals in the non-volatile memorycircuits C1 to C3. In FIG. 1, writing in the volatile memory circuits A1to A3 or B1 to B3 is completed before the reading TFTs 111 to 113 areturned ON to display an image. When data is to be written in thenon-volatile memory circuits, the reading TFTs 111 to 113 are turned OFFand the memory circuit selecting units 120-122 respectively select thenon-volatile memory circuits C1 to C3. No image is displayed on thescreen in this writing period but the writing time is about several msto 100 ms, hardly causing a problem.

Upon turning the display device on, an image is displayed by reading thedigital video signals stored in the non-volatile memory circuits C1 toC3. In this case also, it is desirable to write data in the volatilememory circuits A1 to A3 or B1 to B3 once and read the stored data fromthe volatile memory circuits A1 to A3 or B1 to B3 in the subsequentframe periods.

The above operations are repeated to display an image continuously. Ifthe image to be displayed is a still image, digital video signals arestored in the volatile memory circuits A1 to A3 in the first operation.Once the digital video signals are stored, the digital video signalsstored in the volatile memory circuits A1 to A3 are repeatedly read outfor every new frame period. Thus driving of an external circuit, thesource signal line driver circuit, and the like can be stopped while thestill image is displayed.

Furthermore, digital video signals can be kept stored after power supplyto the liquid crystal display device is shut off by writing digitalvideo signals in the non-volatile memory circuits C1 to C3 provided inthe pixel. Accordingly, when the display device is turned on again, astill image can be displayed without newly sampling the digital videosignals.

Moreover, the gate signal lines can be used one by one, as opposed todriving all of them at once, in writing digital video signals in thememory circuits or reading digital video signals out of the memorycircuits. In other words, partial rewriting of a screen is possible byoperating the source signal line driver circuit and the gate signal linedriver circuit for only a short period of time, thereby increasingdisplay manner options. In this case, it is desirable to use a decoderas the gate signal line driver circuit. A decoder appropriate to use isa circuit disclosed in Japanese Patent Application Laid-open No. Hei8-101609. An example of the decoder is shown in FIG. 20. The sourcesignal line driver circuit may also include a decoder to rewrite a partof a screen.

In this embodiment mode, one pixel has volatile memory circuits A1 to A3and B1 to B3 in order to store 3 bit digital video signals correspondingto two frames. However, the number of memory circuits according to thepresent invention is not limited thereto. For example, when n bitdigital video signals corresponding to m frames are to be stored, onepixel has n×m volatile memory circuits.

Similarly, the number of non-volatile memory circuits according to thepresent invention is not limited by this embodiment mode in which thenon-volatile memory circuits C1 to C3 are provided in one pixel in orderto store 3 bit digital video signals of one frame. If n bit digitalvideo signals of k frames are to be kept stored after power supply isshut off, n×k non-volatile memory circuits are placed in each pixel.

The memory circuits provided in the pixels store digital video signalsin the manner described above, so that the digital video signals storedin the memory circuits can be used repeatedly for every new frame periodwhen a still image is displayed. This makes it possible to continuouslydisplay a still image without driving an external circuit, the sourcesignal line driver circuit, and other circuits. Accordingly theinvention greatly contributes to reduction of power consumption inliquid crystal display devices.

The source signal line driver circuit may not necessarily be formed onan insulator integrally, considering arrangement of the latch circuitsthat increase in number in accordance with the bit number. A part of, orthe entirety of, the source signal line driver circuit may be externalto the insulator.

Although the source signal line driver circuit in this embodiment modeis provided with a number of latch circuits in accordance with the bitnumber, the source signal line driver circuit can operate also when thelatch circuits are provided in a number necessary for only one bit dataprocessing. In this case, digital video signals of from significant bitto less significant bit are inputted to the latch circuits in series.Embodiments of the present invention will be described below.

Embodiment 1

This embodiment gives descriptions on the memory circuit selecting unitsin the circuit shown in Embodiment Mode, regarding its specificstructure (arrangement of transistors and other components) and itsoperation.

FIG. 4 shows a pixel similar to the one shown in FIG. 1, but circuitsconstituting the memory circuit selecting units and the peripheralcircuits thereof are shown here. In FIG. 4, volatile memory circuits A1to A3 and B1 to B3 are respectively connected to writing selecting TFTs420, 422, 424, 426, 428, and 430, and to reading selecting TFTs 421,423, 425, 427, 429, and 431. The volatile memory circuits arerespectively controlled by memory circuit selecting signal lines 414 to419. Non-volatile memory circuits C1 to C3 are connected to writingselecting TFTs 435, 437, and 439, respectively, and are connected toreading selecting TFTs 436, 438, and 440, respectively. The non-volatilememory circuits are respectively controlled by memory circuit selectingsignal lines 432 to 434 and 441 to 443. The pixel shown in thisembodiment stores 3 bit digital video signals corresponding to twoframes in the volatile memory circuits A1 to A3 and B1 to B3, and stores3 bit digital video signals corresponding to one frame in thenon-volatile memory circuits C1 to C3.

The circuit of this embodiment, shown in FIG. 4, may be driven inaccordance with the timing charts described in Embodiment Mode withreference to FIGS. 3A to 3C. The operation of the circuit, plus a methodof actually driving the memory circuit selecting units, will bedescribed referring to FIGS. 3A to 3C and FIG. 4. The description adoptsthe reference symbols used in FIGS. 3A to 3C and FIG. 4.

Reference is made to FIGS. 3A and 3B. In FIG. 3A, frame periods arerespectively denoted by α, β, γ, and δ. The operation of the circuit inthe frame period α is described first.

Shift register circuits, first latch circuits, and second latch circuitsare driven the same way as Embodiment Mode, so see the descriptions ofEmbodiment Mode.

First, pulses are inputted to the memory circuit selecting signal lines414 to 416 to turn the writing selecting TFTs 420, 424, and 428 ON,thereby readying the volatile memory circuits A1 to A3 for writing in.In the period I, pulses are inputted to a writing gate signal line 402to turn a writing TFT 408 conductive and digital video, signals arewritten in the volatile memory circuit A1. Subsequently, in the periodII, pulses are inputted to a writing gate signal line 403 to turn awriting TFT 409 conductive and digital video signals are written in thevolatile memory circuit A2. Lastly, in the period III, pulses areinputted to a writing gate signal line 404 to turn a writing TFT 410conductive and digital video signals are written in the volatile memorycircuit A3.

The above steps complete processing of digital video signalscorresponding to one horizontal period. The periods in FIG. 3Bcorrespond to the period indicated by * in FIG. 3A. The above operationis repeated until the last stage is processed, thereby completingwriting digital video signals of one frame in the volatile memorycircuits A1 to A3.

Subsequently, the operation of the circuit in the frame period β will beexplained. When writing in the volatile memory circuits in the laststage is finished, the first frame is displayed. FIG. 3C is a diagramillustrating a 3 bit time gray scale method. Currently, digital videosignals are assorted by their bits into three groups and the groups arerespectively stored in the volatile memory circuits A1 to A3. Ts1denotes a period of display by first bit data, Ts2 denotes a period ofdisplay by second bit data, and Ts3 denotes a period of display by thirdbit data. The lengths of these display periods are set to satisfyTs1:Ts2:Ts3=4:2:1.

If the display period is divided into periods in accordance with otherrules than power of 2, it is still possible to obtain gray scaledisplay.

Since the signals here are 3 bit signals, the luminance obtained rangesfrom 0 to 7, namely, 8 levels in total. When a pixel is not brought intoan ON state in any of the periods Ts1 to Ts3, the luminance is 0. When apixel is in an ON state throughout the entire periods, the luminance is7. If a pixel is to have the fifth level of luminance, the pixel isbrought into ON state in Ts1 and Ts3.

Specific explanations will be given referring to the drawings. Whenmoving on to the display period after the operation of writing data inthe volatile memory circuits is finished, supply of pulses to the memorycircuit selecting signal lines 414 to 416 are cut to turn the writingselecting TFTs 420, 424, and 428 unconductive. At the same time, pulsesare inputted to the memory circuit selecting signal lines 417 to 419 toturn the reading selecting TFTs 421, 425, and 429 conductive, therebyreadying the volatile memory circuits A1 to A3 for reading. In theperiod Ts1, pulses are inputted to a reading gate signal line 405 toturn a reading TFT 411 conductive so that the pixel is driven inaccordance with the digital video signals stored in the volatile memorycircuit A1 Subsequently, in Ts2; pulses are inputted to a reading gatesignal line 406 to turn a reading TFT 412 conductive so that the pixelis driven in accordance with the digital video signals stored in thevolatile memory circuit A2. Lastly, in Ts3, pulses are inputted to areading gate signal line 407 to turn a reading TFT 413 conductive sothat a voltage is applied to the pixel in accordance with the digitalvideo signals stored in the volatile memory circuit A3.

Data corresponding to one frame period are displayed as above.Meanwhile, the driver circuit is processing digital video signals of thenext frame period. The procedure is the same as the one described aboveup through transferring the digital video signals to the second latchcircuits. In the subsequent memory circuit writing period, the volatilememory circuits B1 to B3 are used.

During the period in which signals are written in the volatile memorycircuits A1 to A3, the writing selecting TFTs 420, 424, and 428 areturned conductive to make the volatile memory circuits A1 to ASwriteable and, at the same time, the reading selecting TFTs 423, 427,and 431 are turned conductive to make the volatile memory circuits B1 toB3 readable. On the other hand, during the period in which signals arewritten in the volatile memory circuits B1 to B3. the writing selectingTFTs 422, 426, and 430 are turned conductive to make the volatile memorycircuits B1 to B3 writeable and, at the same time, the reading selectingTFTs 421, 425, and 429 are turned conductive to make the volatile memorycircuits A1 to A3 readable. To summarize, the volatile memory circuitsA1 to A3 and B1 to B3 in the pixel of this embodiment alternatelyswitches between writing and reading over every new frame period.

In the period I, pulses are inputted to the writing gate signal line 402to turn the writing TFT 408 conductive so that digital video signals arewritten in the volatile memory circuit B1. Subsequently, in the periodII, pulses are inputted to the writing gate signal line 403 to turn thewriting TFT 409 conductive so that digital video signals are written inthe volatile memory circuit B2. Lastly, in the period III, pulses areinputted to the writing gate signal line 404 to turn the writing ITT 410conductive so that digital video signals are written in the volatilememory circuit B3.

Then the frame period γ is started and the second frame is displayed inaccordance with the digital video signals stored in the volatile memorycircuits B1 to B3. At the same time, processing of digital video signalsof the next frame period is commenced. The digital video signals of thenext frame period are stored again in the volatile memory circuits A1 toA3 that have finished their operations related to display of the firstframe period.

Thereafter, the digital video signals stored in the volatile memorycircuits A1 to A3 are displayed in the frame period δ, and digital videosignals of the next frame period simultaneously begin to receiveprocessing. The digital video signals of the next frame period arestored again in the volatile memory circuits B1 to B3 that have finishedtheir operations related to display of the second frame period.

The operation of writing digital video signals in the non-volatilememory circuits C1 to C3 and the operation of reading digital videosignals out of the non-volatile memory circuits C1 to C3 are the same asEmbodiment Mode.

The procedure above is repeated to display an image. When the image tobe displayed is a still image, the source signal line driver circuitstops its operation after digital video signals of some frame arewritten in the memory circuits, and the image is displayed by readingthe same signals stored in the memory circuits each time a new frame isstarted. In this way, power consumption during still image display canbe reduced greatly. Moreover, by storing the digital video signals usingthe non-volatile memory circuits, the digital video signals of the stillimage are kept stored after power supply to the display device is shutoff and hence the still image can be displayed next time the displaydevice is turned on.

Embodiment 2

This embodiment gives a description on a case where signals are writtenin volatile memory circuits of a pixel portion by dot-sequential systemto eliminate the need for a second latch circuit of a source signal linedriver circuit.

FIG. 5 shows the structure of a source signal line driver circuit andthe structure of some of pixels in a liquid crystal display device thatemploys pixels having memory circuits. The circuit is capable ofhandling 3 bit digital video signals, and is composed of shift registercircuits 501, latch circuits 502, and pixels 503. Denoted by 510 aresignal lines to which signals are inputted from a gate signal linedriver circuit or directly from the external, and descriptions of thesignal lines will be found later along with explanations of the pixels.

FIG. 6 shows a detailed circuit structure of one of the pixels 503 inFIG. 5. As in Embodiment 1, the pixel is for 3 bit digital gray scales,and is composed of a liquid crystal element (LC), volatile memorycircuits (A1 to A3 and B1 to B3), non-volatile memory circuits (C1 toC3), etc. Denoted by 601 is a first bit (MSB) signal source signal line,602, a second bit signal source signal line, and 603, a third bit (LSB)signal source signal line. Reference symbol 604 represents a writinggate signal line, 605 to 607, reading gate signal lines, 608 to 610,writing TFTs, and 611 to 613, reading TFTs. Memory circuit selectingunits include writing selecting TFTs 620, 622, 624, 626, 628, and 630,reading selecting TFTs 621, 623, 625, 627, 629, and 631, and othercircuits, 632 to 634 and 641 to 643 denote memory circuit selectingsignal lines. Memory circuit selecting units for the non-volatile memorycircuits C1 to C3 include writing selecting TFTs 636, 638, and 640,reading selecting TFTs 635, 637, and 639, and other circuits.

FIGS. 7A to 7C are timing charts regarding driving of the circuit ofthis embodiment. The description will be given with reference to FIG. 5,FIG. 6, and FIGS. 7A to 7C.

The operation of the shift register circuits 501 and the latch circuits(LAT1) 502 is the same as Embodiment Mode and Embodiment 1. As shown inFIG. 7B, writing in the volatile memory circuits in the pixel is startedimmediately after the latch operation for the first stage is finished.Pulses are inputted to the writing gate signal line 604 to turn thewriting TFTs 608 to 610 conductive and the memory circuit selectingsignal lines 614 to 616 also receive pulses to turn the writingselecting TFTs 620, 624, 628 conductive. The volatile memory circuits A1to A3 are thus readied for writing. The digital video signals grouped bytheir bits and held in the latch circuits 502 in groups aresimultaneously written in the volatile memory circuits through the threesource signal lines 601 to 603.

While the digital video signals held in the latch circuits are writtenin the volatile memory circuits in the first stage, sampling pulses arenewly outputted and, in response, digital video signals for the nextstage are held in the latch circuits. Signals are thus sequentiallywritten in the volatile memory circuits.

The operation above is conducted in one horizontal period (correspondingto the period indicated by ** in FIG. 7A) and the same operation isrepeated for all the columns to complete writing digital video signalsof one frame in the volatile memory circuits in the frame period α. Thena display period for the first frame, namely, the frame period β, isstarted. Supply of pulses to the writing gate signal line 604 is cut andsupply of pulses to the memory circuit selecting signal lines 614 to 616is also stopped to turn the writing selecting TFTs 620, 624, and 628unconductive. Instead, pulses are inputted to the memory circuitselecting signal lines 617 to 619 to turn the reading selecting TFTs621, 625, and 629 conductive, thereby readying the volatile memorycircuits A1 to A3 for reading.

The time gray scale method described in Embodiment 1 is then applied asshown in FIG. 7C. In the display period Ts1, pulses are inputted to areading gate signal line 605 to turn a reading TFT 611 conductive sothat the digital video signals stored in the volatile memory circuit A1are read out for display. Subsequently, in Ts2, pulses are inputted to areading gate signal line 606 to turn a reading TFT 612 conductive sothat the digital video signals stored in the volatile memory circuit A2are read out for display. Similarly, in Ts3, pulses are inputted to areading gate signal line 607 to turn a reading TFT 613 conductive sothat the digital video signals stored in the volatile memory circuit A3are read out for display.

Thus a display period for the first frame is completed. In the frameperiod β, digital video signals of the next frame are beginning to beprocessed at the same time. The procedure is the same as the onedescribed above up through holding the digital video signals in thelatch circuits 502. In the subsequent volatile memory circuit writingperiod, the volatile memory circuits B1 to B3 are used.

During the period in which signals are written in the volatile memorycircuits A1 to A3, the writing selecting TFTs 620, 624, and 628 areturned conductive to make the volatile memory circuits A1 to A3writeable and, at the same time, the reading selecting TFTs 623, 627,and 631 are turned conductive to make the volatile memory circuits B1 toB3 readable. On the other hand, during the period in which signals arewritten in the volatile memory circuits B1 to B3, the writing selectingTFTs 622, 626, and 630 are turned conductive to make the volatile memorycircuits B1 to B3 writeable and, at the same time, the reading selectingTFTs 621, 625, and 629 are turned conductive to make the volatile memorycircuits A1 to A3 readable. To summarize, the volatile memory circuitsA1 to A3 and B1 to B3 in the pixel of this embodiment alternatelyswitches between writing and reading over every new frame period.

The operation of writing in the volatile memory circuits B1 to B3 andthe operation of reading out of the volatile memory circuits B1 to B3are the same as the volatile memory circuits A1 to A3. When writingsignals in the volatile memory circuits B1 to B3 is finished, the frameperiod γ is started to move on to the display period for the secondframe. Concurrently, digital video signals of the next frame period isprocessed in the frame period γ. The procedure is the same as the onedescribed above up through holding the digital video signals in thelatch circuits 502. In the subsequent volatile memory circuit writingperiod, the volatile memory circuits A1 to A3 are again used.

Thereafter, the digital video signals stored in the volatile memorycircuits A1 to A3 are displayed in the frame period δ, and digital videosignals of the next frame period simultaneously begin to receiveprocessing. The digital video signals of the next frame period arestored again in the volatile memory circuits B1 to B3 that have finishedtheir operations related to display of the second frame period.

The operation of writing digital video signals in the non-volatilememory circuits C1 to C3 and the operation of reading digital videosignals out of the non-volatile memory circuits C1 to C3 are the same asEmbodiment Mode.

The procedure above is repeated to display an image. When the image tobe displayed is a still image, the source signal line driver circuitstops its operation after digital video signals of some frame arewritten in the memory circuits, and the image is displayed by readingthe same signals stored in the memory circuits each time a new frameperiod is started. In the case where a still image is displayed afterpower supply to the display device is shut off once and then the displaydevice is turned on, digital video signals stored in the non-volatilememory circuits C1 to C3 are read to display the still image. In thisway, power consumption during still image display can be reducedgreatly. Furthermore, the number of latch circuits is reduced to halfthe number of latch circuits in Embodiment 1. This embodiment istherefore space-saving in arrangement of the circuits, and cancontribute to overall size reduction of the display device.

Embodiment 3

This embodiment describes an example of a liquid crystal display deviceto which the circuit structure of the liquid crystal display deviceshown in Embodiment 2 and having no second latch circuit is applied, andwhich employs dot-sequential driving to write signals in memory circuitsin each pixel.

FIG. 17 shows an example of the circuit structure for a source signalline driver circuit of a liquid crystal display device according to thisembodiment. The circuit is capable of handling 3 bit digital gray scalesignals, and is composed of shift register circuits 1701, latch circuits1702, switching circuits 1703, and pixels 1704. Denoted by 1710 aresignal lines to which signals are supplied from a gate signal linedriver circuit, or directly from the external. The circuit structure ofthe pixels is the same as Embodiment 2, and hence FIG. 6 can be referredto as it is.

FIGS. 18A to 18C are timing charts regarding driving of the circuit ofthis embodiment. The description will be given with reference to FIG. 6,FIG. 17 and FIGS. 18A to 18C.

The operations from outputting sampling pulses from the shift registercircuits 1701 through holding digital video signals in the latchcircuits 1702 in response to the sampling pulses are the same asEmbodiments 1 and 2. In this embodiment, the switching circuits 1703 areplaced between the latch circuits 1702 and the volatile memory circuitsin the pixels 1704. Therefore writing in the volatile memory circuitsdoes not start immediately after completing holding the digital videosignals in the latch circuits. The switching circuits 1703 are keptclosed until the dot data sampling period is ended, and the latchcircuits continue to hold the digital video signals as long as theswitching circuits are closed.

As shown in FIG. 18B, the switching circuits 1703 are opened all at onceupon receiving input of latch signals (latch pulses) during the retraceperiod that follows completion of holding digital video signalscorresponding to one horizontal period. Then the digital video signalsheld in the latch circuits 1702 are simultaneously written in thevolatile memory circuits in the pixels 1704. The operation in the pixels1704 during this writing operation, and the operation in the pixels 1704during reading operation for display in the next frame period are thesame as Embodiment 2, and hence explanations thereof are omitted here.Also, how and when to write data in non-volatile memory circuits is thesame as Embodiment 2, and hence explanations thereof will not berepeated.

In this way, driving in accordance with dot-sequential system can easilybe made also when a source signal line driver circuit has no secondlatch circuit.

Embodiment 4

In Embodiment 4, a method of manufacturing TFTs of a pixel portion, adriver circuit portion (source signal line driver circuit, gate signalline driver circuit and pixel selection signal line driven circuit)formed in the periphery thereof in an active EL display device of thepresent invention simultaneously and a non-volatile storage circuit atthe same time is explained. Note that a CMOS circuit which is a baseunit is illustrated as the driver circuit portion to make a briefexplanation.

First, as shown in FIG. 9A, a base film 5002 made from an insulatingfilm such as a silicon oxide film, a silicon nitride film, or a siliconnitride oxide film is formed on a substrate 5001 made from glass such asbarium borosilicate glass or aluminum borosilicate glass, typicallyCorning Corp. #7059 glass or #1737 glass. For example, a silicon nitrideoxide film 5002 a made from SiH₄, NH₃, and N₂O by plasma CVD is formedwith a thickness of 10 to 200 nm (preferably from 50 to 100 nm), and ahydrogenized silicon nitride oxide film 5002 b with a thickness of 50 to200 nm (preferably between 100 and 150 nm), made from SiH₄ and N₂O, issimilarly formed and laminated. The base film 5002 is shown as a twolayer structure in Embodiment 4, but it may also be formed as a singlelayer of the above insulating films, and it may also be formed having alamination structure in which two layers or more are laminated.

Island shape semiconductor layers 5003 to 5006 are formed by acrystalline semiconductor film manufactured using a lasercrystallization method of a semiconductor film having an amorphousstructure, or using a known thermal crystallization method. Thethickness of the island-shape semiconductor layers 5003 to 5006 isformed to a thickness of 25 to 80 nm (preferably between 30 and 60 nm).There are no limitations in the crystalline semiconductor film material,but it is preferable to form the film from a silicon or a silicongermanium (SiGe) alloy.

A laser such as a pulse emission type or continuous emission typeexcimer laser, a YAG laser, and a YVO₄ laser can be used in the lasercrystallization method to manufacture a crystalline semiconductor film.A method of condensing laser light emitted from a laser oscillator intoa linear shape by an optical system and then irradiating the light tothe semiconductor film may be used when these types of lasers are used.The crystallization conditions may be suitably selected by the operator,but when using the excimer laser, the pulse emission frequency is set to30 Hz, and the laser energy density is set from 100 to 400 mJ/cm²(typically between 200 and 300 mJ/cm²). Further, the second harmonic isutilized when using the YAG laser, the pulse emission frequency is setfrom 1 to 10 KHz, and the laser energy density may be set from 300 to600 mJ/cm² (typically between 350 and 300 mJ/cm²). The laser lightcollected into a linear shape with a width of 100 to 1000 μm, forexample 400 μm, is then irradiated over the entire surface of thesubstrate. This is performed with an overlap ratio of 80 to 98% for thelinear shape laser light.

A gate insulating film 5007 is formed covering the semiconductor layers5003 to 5006. A gate insulating film 5007 is formed by an insulatingfilm containing silicon with a thickness of 40 to 150 nm by plasma CVDor sputtering. A 120 nm thick silicon nitride oxide film is formed inEmbodiment 4. The gate insulating film is not limited to this type ofsilicon nitride oxide film, of course, and other insulating filmscontaining silicon may also be used, in a single layer or in alamination structure. For example, when using a silicon oxide film, itcan be formed by plasma CVD with a mixture of TEOS (tetraethylorthosilicate) and O₂, at a reaction pressure of 40 Pa, with thesubstrate temperature set from 300 to 400° C., and by discharging at ahigh frequency (13.56 MHz) electric power density of 0.5 to 0.8 W/cm².Good characteristics as a gate insulating film can be obtained bysubsequently performing thermal annealing, at between 400 and 500° C.,of the silicon oxide film thus manufactured.

A first conductive film 5008 and a second conductive film 5009 are thenformed on the gate insulating film 5007 in order to form gateelectrodes. The first conductive film 5008 is formed from Ta (tantalum)with a thickness of 50 to 100 nm, and the second conductive film 5009 isformed from W (tungsten) having a thickness of 100 to 300 nm, inEmbodiment 4.

The Ta film is formed by sputtering of a Ta target by Ar. If appropriateamounts of Xe and Kr are added to Ar at the time of sputtering, theinternal stress of the Ta film is relaxed, and film peeling can beprevented. The resistivity of an α phase Ta film is on the order of 20μΩcm, and it can be used in the gate electrode, but the resistivity of aβ phase Ta film is on the order of 180 μΩcm and it is unsuitable for thegate electrode. An α phase Ta film can easily be obtained if a tantalumnitride film, which possesses a crystal structure near that of α phaseTa, is formed with a thickness of about 10 to 50 nm as a base for Ta inorder to form α phase Ta.

The W film is formed by sputtering with a W target, which can also beformed, by thermal CVD using tungsten hexafluoride (WF₆). Whichever isused, it is necessary to be able to make the film become low resistancein order to use it as the gate electrode, and it is preferable that theresistivity of the W film be made equal to or less than 20 μΩcm. Theresistivity can be lowered by enlarging the crystal grains of the Wfilm, but for cases in which there are many impurity elements such asoxygen in the W film, crystallization is inhibited, and the film becomeshigh resistance. A W target having a purity of 99.9999% is thus used insputtering. In addition, by forming the W film while taking sufficientcare that no impurities from within the gas phase are introduced at thetime of film formation, a resistivity of 9 to 20 μΩcm can be achieved.

Note that although the first conductive film 5008 is Ta and the secondconductive film 5009 is W in Embodiment 4, the conductive films are notlimited to these, and both may also be formed from an element selectedfrom the group consisting of Ta, W, Ti, Mo, Al, and Cu, or from an alloymaterial having one of these elements as its main component, or from achemical compound of these elements. Further, a semiconductor film,typically a polysilicon film into which an impurity element such asphosphorous is doped, may also be used. Examples of preferablecombinations other than that used in Embodiment 4 include: forming thefirst conductive film 5008 by tantalum nitride (TaN) and combining itwith the second conductive film 5009 formed from W; forming the firstconductive film 5008 by tantalum nitride (TaN) and combining it with thesecond conductive film 5009 formed from Al; and forming the firstconductive film 5008 by tantalum nitride (TaN) and combining it with thesecond conductive film 5009 formed from Cu.

Then, a mask 5010 is formed from resist, and a first etching process isperformed in order to form electrodes and wirings. An ICP (inductivelycoupled plasma) etching method is used in Embodiment 4. A gas mixture ofCF, and Cl, is used as an etching gas, and a plasma is generated byapplying a 500 W RF electric power (13.56 MHz) to a coil shape electrodeat a pressure of 1 Pa. A 100 W RF electric power (13.56 MHz) is alsoapplied to the substrate side (test piece stage), effectively applying anegative self-bias voltage. The W film and the Ta film are both etchedon the same order when CF, and Cl₂ are combined.

The edge portions of the first conductive layer and the secondconductive layer are made into a tapered shape in accordance with theeffect of the bias voltage applied to the substrate side under the aboveetching conditions by using a suitable resist mask shape. The angle ofthe tapered portions is from 15 to 45°. The etching time may beincreased by approximately 10 to 20% in order to perform etching withoutany residue remaining on the gate insulating film. The selectivity of asilicon nitride oxide film with respect to a W film is from 2 to 4(typically 3), and therefore approximately 20 to 50 nm of the exposedsurface of the silicon nitride oxide film is etched by this over-etchingprocess. In this way, the first shape conductive layers 5011 to 5016 arethus formed from the first conductive layers and the second conductivelayers (the first conductive layers 5011 a to 5016 a and the secondconductive layers 5011 b to 5016 b) in accordance with the first etchingprocess. At this time, regions of the gate insulating film 5007 notcovered by first shape conductive layers 5011 to 5016 are made thinnerby about 20 to 50 nm. (FIG. 9B)

A first doping process is then performed, and an impurity element whichimparts n-type conductivity is added. The doping can be carried out byion doping or ion implantation. Ion doping is performed under conditionsof a dose amount from 1×10¹³ to 5×10¹⁴ atoms/cm² and an accelerationvoltage of 60 to 100 keV. An element in periodic table group 15,typically phosphorous (P) or arsenic (As) is used as the impurityelement which imparts n-type conductivity, and phosphorous (P) is usedhere. The conductive layers 5011 to 5016 become masks with respect tothe n-type conductivity imparting impurity element in this case, and thefirst impurity regions 5017 to 5020 are formed in a self-aligningmanner. The impurity element which imparts n-type conductivity is addedto the first impurity regions 5017 to 5020 at a concentration in therange of 1×10²⁰ to 1×10<atoms/cm³. (FIG. 9B)

Next, as shown in FIG. 9C, a second etching process is performed withoutremoving the resist mask. The etching gas of the mixture of CF₄, Cl₂ andO₂ is used, and the W film is selectively etched. At this point, secondshape conductive layers 5021 to 5026 (first conductive layers 5021 a to5026 a and second conductive layers 5021 b to 5026 b) are formed by thesecond etching process. Regions of the gate insulating film 5007, whichare not covered with the second shape conductive layers 5021 to 5026 aremade thinner by about 20 to 50 nm by etching.

An etching reaction of the W film or the Ta film by the mixture gas ofCF₄ and Cl₂ can be guessed from a generated radical or ion species andthe vapor pressure of a reaction product. When the vapor pressures offluoride and chloride of W and Ta are compared with each other, thevapor pressure of WF₆ of fluoride of W is extremely high, and otherWCl₅, TaP₅, and TaCl₅ have almost equal vapor pressures. Thus, in themixture gas of CF, and Cl₂, both the W film and the Ta film are etched.However, when a suitable amount of O₂ is added to this mixture gas, CF₄and O₂ react with each other to form CO and F, and a large number of Fradicals or F ions are generated. As a result, an etching rate of the Wfilm having the high vapor pressure of fluoride is increased. On theother hand, with respect to Ta, even if F is increased, an increase ofthe etching rate is relatively small. Besides, since Ta is easilyoxidized as compared with W, the surface of Ta is oxidized by additionof O₂. Since the oxide of Ta does not react with fluorine or chlorine,the etching rate of the Ta film is further decreased. Accordingly, itbecomes possible to make a difference between the etching rates of the Wfilm and the Ta film, and it becomes possible to make the etching rateof the W film higher than that of the Ta film.

Then, as shown in FIG. 10A, a second doping process is performed. Inthis case, a dosage is made lower than that of the first doping processand under the condition of a high acceleration voltage, an impurityelement for imparting the n-type conductivity is doped. For example, theprocess is carried out with an acceleration voltage set to 70 to 120 keVand at a dosage of 1×10¹³ atoms/cm², so that new impurity regions areformed inside of the first impurity regions formed into the island-shapesemiconductor layers in FIG. 10B. Doping is carried out such that thesecond shape conductive layers 5021 to 5026 are used as masks to theimpurity element and the impurity element is added also to the regionsunder the first conductive layers 5021 a to 5026 a. In this way, secondimpurity regions 5027 to 5031 are formed. The concentration ofphosphorous (P) added to the third impurity regions 5027 to 5031 has agentle concentration gradient in accordance with the thickness oftapered portions of the first conductive layers 5021 a to 5026 a. Notethat in the semiconductor layer that overlap with the tapered portionsof the first conductive layers 5021 a to 5026 a, the impurityconcentration slightly falls from the end portions of the taperedportions of the first conductive layers 5021 a to 5026 a toward theinner portions, but the concentration keeps almost the same level.

As shown in FIG. 10B, a third etching process is performed using theetching gas of CHF₆, and a reactive ion etching (RIE method) is used. Bythe third etching process, the tapered portion of the first conductivelayers 5021 a to 5026 a are partly etched to contract the overlappingregion of the first conductive layers with a semiconductor layer. Thirdshape conductive layers 5037 to 5040 (first conductive layers 5032 a to5037 a and second conductive layers 5032 b to 5037 b) are formed by thethird etching process. Regions of the gate insulating film 5007, whichare not covered with the third shape conductive layers 5032 to 5037 aremade thinner by about 20 to 50 nm by etching.

By the third etching process, second impurity regions 5027 a to 5013 athat overlap with the first conductive layers 5032 a to 5037 a and thirdimpurity regions 5027 b to 5031 b between the first impurity region andthe third impurity region, are formed in the second impurity regions5027 to 5031.

Fourth impurity regions 5039 to 5044 having a conductivity type which isthe opposite of the first conductive type, are then formed as shown inFIG. 10C in the island shape semiconductor layer 5004 which formsp-channel TFTs. The third shape conductive layer 5033 b is used as amask with respect to the impurity element, and the impurity regions areformed in a self-aligning manner. The island shape semiconductor layers5003 and 5005, which form n-channel TFTs, are covered over their entiresurface areas by resist masks 5038. Phosphorous is added in differingconcentration to the impurity regions 5039 to 5044, and ion doping isperformed here using diborane (B₂H₆), so that the impurity concentrationin the regions becomes from 2×10²⁰ to 2×10²¹ atoms/cm³.

Impurity regions are formed in the respective island shape semiconductorlayers by the above processes. The third shape conductive layers 5032,5033, 5035 and 5036 overlapping the island shape semiconductor layersfunction as gate electrodes. Reference numeral 5037 functions asfloating gate electrode of the memory cell. Also reference numeral 5034functions island shape source signal line. (FIG. 10C)

After removing the resist mask 5038, the second gate insulating film isformed as shown in FIG. 11A. A thickness of the second gate insulatingfilm 5045 may be 10 to 250 nm. It can be using known gas phase method(plasma CVD or sputtering and so on) as a method of film formation. Notthat SiNO film is formed by plasma CVD having a thickness of 70 nm, inEmbodiment 4.

A process of activating the impurity elements added to the respectiveisland shape semiconductor layers is then performed with the aim ofcontrolling conductivity type. Thermal annealing using an annealingfurnace is performed for this process. In addition, laser annealing andrapid thermal annealing (RTA) can also be applied. Thermal annealing isperformed with an oxygen concentration equal to or less than 1 ppm,preferably equal to or less than 0.1 ppm, in a nitrogen atmosphere at400 to 700° C., typically between 500 and 600° C. Heat treatment isperformed for 4 hours at 500° C. in Embodiment 4. However, for cases inwhich the wiring material used in the third shape conductive layers 5037to 5040 is weak with respect to heat, it is preferable to performactivation after forming an interlayer insulating film (having siliconas its main constituent) in order to protect the wirings and the like.

In addition, heat treatment is performed for 1 to 12 hours at 300 to450° C. in an atmosphere containing between 3 and 100% hydrogen,performing hydrogenation of the island shape semiconductor layers. Thisprocess is one of terminating dangling bonds in the island shapesemiconductor layers by hydrogen which is thermally excited. Plasmahydrogenation (using hydrogen excited by a plasma) may also be performedas another means of hydrogenation.

Thereafter, a conductive film with a thickness of 200 to 400 nm isformed and patterned to form a control gate electrode 5046. The control-gate electrode 5046 is formed such that it partially or entirelyoverlaps a floating gate electrode 5037 with the second gate insulatingfilm 5045 sandwiched therebetween (FIG. 11A).

A first interlayer insulating film 5047 is formed from a siliconoxynitride film having a thickness of 100 to 200 nm. Formed next aresource wiring lines 5048 and 5050 contacting the source region of theisland-like semiconductor layer in the driver circuit portion, and adrain wiring line 5049 contacting the drain region thereof. In the pixelportion, connector electrodes 5051 and 5052 are formed. At the sametime, connector electrodes 5053 and 5054 are formed in the memory cellportion. The connector electrode 5051 forms an electrical connectionbetween the source signal line 5034 and the pixel TFT (FIG. 11B).

A second interlayer insulating film 5055 is formed thereon from anorganic insulating material, and a pixel electrode 5056 is formed nextWhen the display device to be manufactured is a reflective liquidcrystal display device as the one shown in this embodiment, the pixelelectrode 5056 is desirably formed of a material having highreflectance, such as a film mainly containing Al or Ag or a laminate ofa Al film and a Ag film.

In the manner described above, a driver circuit portion having ann-channel TFT and a p-channel TFT can be formed on the same substrate onwhich a pixel portion having a pixel TFT and volatile memory circuits isformed as shown in FIG. 11C. A substrate such as this is called anactive matrix substrate in this specification.

After the active matrix substrate in the state of FIG. 11C is obtained,an oriented film 5057 is formed on the active matrix substrate andsubjected to rubbing treatment as shown in FIG. 12A.

An opposite substrate 5058 is prepared. On the opposite substrate 5058,an opposite electrode 5059 is formed by patterning and an oriented film5060 is formed and subjected to rubbing treatment. The oppositeelectrode is formed of an ITO film or like other transparent conductivematerials.

A spacer (not shown) is formed on the active matrix substrate or theopposite substrate. The spacer may be spherical beads sprayed to thesubstrate. Alternatively, a photosensitive resin is patterned into dotsor stripes in a display region. The spacer prevents orientation defectof a liquid crystal material.

A desirable cell gap for the reflective liquid crystal display device ofthis embodiment is 0.5 to 1.5 μm, considering retardation. In thisembodiment, the cell gap in the pixel portion is set to 1.0 μm.

The active matrix substrate on which the pixel portion and the drivercircuit portion are formed is then bonded to the opposite substrateusing a seal 5061. The seal 5061 has a filler mixed therein, and thefiller, together with the spacer, keeps the distance uniform between thetwo substrates when they are bonded. A liquid crystal material 5062 isinjected between the substrates, and then the substrates are completelysealed by an end sealing material (not shown). The liquid crystalmaterial 5062 may be a known liquid crystal material. Thus completed isan active matrix liquid crystal display device shown in FIG. 12A.

The TFTs have a top gate structure in the active matrix liquid crystaldisplay device manufactured by the above process. However, thisembodiment can readily be applied to cases in which TFTs have a bottomgate structure or other structures.

Although a glass substrate is used in this embodiment, the substrate isnot limited to the glass substrate. This embodiment can be carried outalso with a plastic substrate, a stainless steel substrate, a singlecrystal wafer, and the like.

The description given in this embodiment takes a reflective liquidcrystal display device as an example. However, with a pixel electrodestructured differently, the present invention can readily be applied toa transmissive liquid crystal display device and to a semi-transmissivedisplay device in which half the pixels have reflective electrodes andthe rest of the pixels have transparent electrodes.

Embodiment 5

The display device of the present invention employs the time gray scalemethod to obtain gray scale display. Accordingly, if pixels have liquidcrystal elements, a quicker response is required than the usual analoggray scale method and it is desirable to use a ferroelectric liquidcrystal (FLC). This embodiment describes an example of fabricating asubstrate suitable for a liquid crystal element made of a ferroelectricliquid crystal through the display device manufacture process introducedin Embodiment 4.

An active matrix substrate shown in FIG. 19A (corresponding to FIG. 11C)is fabricated in accordance with Embodiment 4.

An opposite substrate 5058 is prepared. On the opposite substrate 5058,an opposite electrode 5059 is formed by patterning. The oppositeelectrode is formed of an ITO film or like other transparent conductivematerials.

Oriented films 5101 and 5102 are formed on the active matrix substrateand the opposite substrate, respectively. An oriented film RN 1286, aproduct of Nissan Chemical Industries, LTD., is formed and pre-baked at90° C. for five minutes before post-baking it at 250° C. for an hour.The thickness of the film after post-baking is 40 nm. The oriented filmis formed by flexographic printing or spinner application. The RN 1286adheres poorly to a seal, and hence the oriented film is removed fromthe positions where the seal is to be placed. Also, the oriented film isnot formed on a contact pad for electrically connecting the activematrix substrate with the opposite substrate and on a lead wire forconnecting a flexible printed circuit (FPC).

The oriented films 5101 and 5102 are subjected to rubbing treatment. Inthe rubbing treatment, the rubbing directions are set to be parallel toeach other when the opposite substrate 5058 is bonded to the activematrix substrate. Rubbing cloth used in the rubbing treatment is YA-20R,a product of Yoshikawa Chemical. A rubbing apparatus produced by JoyoEngineering, Co., LTD. is used to subject the oriented films to rubbingonce with the depression amount set to 0.25 mm, the roll rpm to 100 rpm,and the stage rate to 10 mm/sec. The diameter of the rubbing roll is 130mm. After the rubbing, the oriented films are washed by running waterover the substrate surfaces.

A seal 5103 is formed next. The seal is patterned into a shape havingone liquid crystal material inlet so that the liquid crystal materialcan be injected in vacuum.

The seal is formed on the opposite substrate using a seal dispenserproduced by Hitach Chemical Co., Ltd. The seal used is XN-21S, a productof Mitsui Chemicals, Inc. The seal is subjected to test baking at 90° C.for thirty minutes and then gradually cooled for the following fifteenminutes.

It is a known fact that the seal XN-21S can provide a cell gap of merely2.3 to 2.6 μm even after subjected to thermal pressing. Then, in orderto obtain a cell gap of 1.0 μm, the seal is arranged in a region wherethe laminate is thinner than the pixel portion by 1.5 μm or more. Inthis embodiment, the seal 5103 is placed in a region where a firstinterlayer insulating film 5047 and a second interlayer insulating film5055 are removed by etching.

A conductive spacer (not shown) is formed at the same time the seal isformed.

The spacer (not shown) is formed on the active matrix substrate or theopposite substrate. The spacer may be spherical beads sprayed to thesubstrate. Alternatively, a photosensitive resin is patterned into dotsor stripes in a display region. The spacer prevents orientation defectof a liquid crystal material.

A desirable cell gap for a reflective liquid crystal display device is0.5 to 1.5 μm, considering retardation. In this embodiment, the cell gapin the pixel portion is set to 1.0 μm.

After that, with a bonding apparatus by Newtom Corporation, the activematrix substrate and the opposite substrate are bonded to each other byaligning markers on the substrates.

The seal is then thermally cured in a clean oven at 160° C. for threehours while applying a pressure of 0.3 to 1.0 kgf/cm² to the entiresurfaces of the substrates in the direction perpendicular to thesubstrate plane. The opposite substrate and the active matrix substrateare thus bonded.

A pair of substrates consisting of the bonded opposite substrate andactive matrix substrate are cut into pieces.

A liquid crystal material 5104 is a bistable ferroelectric liquidcrystal or a tristable ferroelectric liquid crystal.

The liquid crystal material is heated until it reaches the isotropicphase and then injected. Thereafter, the liquid crystal material isgradually cooled at 0.1° C. per minute to room temperature (FIG. 19B).

A UV-curable resin (not shown) as an end-sealing material is applied bya small-sized dispenser so as to cover the liquid crystal materialinlet.

The flexible printed board (not shown) is then bonded using ananisotropic conductive film (not shown) to complete an active matrixliquid crystal display device.

If a pixel electrode on the active matrix substrate is formed of atransparent conductive film, a transmissive liquid crystal displaydevice can also be formed by the manufacture process of this embodiment.A desirable cell gap for the transmissive liquid crystal display deviceis 1.0 to 2.5 μm, considering retardation and avoiding the helicalstructure of the ferroelectric liquid crystal.

Embodiment 6

Static random access memories (SRAM) are used for the volatile memorycircuits in the pixel portions of the liquid crystal display devicesaccording to Embodiments 1 through 3 of the present invention. However,the volatile memory circuits are not limited to SRAM. Dynamic randomaccess memories (DRAM) can be given as other volatile memory circuitsemployable by a pixel portion in a liquid crystal display device of thepresent invention. This embodiment shows an example of using variousmemory circuits.

FIG. 8 shows the case in which DRAM is used for volatile memory circuitsA1 to A3 and B1 to B3 arranged in a pixel. The basic structure thereofis the same as the circuits shown in Embodiment 1. The DRAM used for thevolatile memory circuits A1 to A3 and B1 to B3 may have a generalstructure. In this embodiment, a relatively simple structure comprisedof an inverter and a capacitor storage as shown in FIG. 8 is used.

The operation of a source signal line driver circuit is the same asEmbodiment 1. Unlike SRAM, DRAM requires rewriting for once in a while(the rewriting operation is hereinafter referred to as refresh), andtherefore is connected to refresh TFTs 801 to 803. Refresh is achievedby turning the refresh TFTs 801 to 803 conductive at some point in astill image display period (the period during which digital videosignals stored in the volatile memory circuits are repeatedly read outto display a still image), so that electric charges in a pixel portionis returned to the volatile memory circuit side.

Still another format of memory circuits that can be used to constitute apixel portion in a liquid crystal display device of the presentinvention is, though not shown in the drawing, ferroelectric randomaccess memory (FeRAM). FeRAM is a non-volatile memory and having thesame level of writing speed as SRAM and DRAM. Characteristics of FeRAM,including low writing voltage, can be utilized to further reduce powerconsumption of the liquid crystal display device of the presentinvention. Flash memories may also be used to constitute the memorycircuits of the present invention.

Embodiment 7

An active matrix semiconductor display device made from a driver circuitof the present invention has various uses. In the present embodiment, adescription will be given on a semiconductor device incorporating anactive matrix semiconductor display device made from a driver circuit ofthe present invention.

The following can be given as examples of these semiconductor devices: aportable information terminal (such as an electronic book, a mobilecomputer, and a portable telephone), a video camera, a digital camera, apersonal computer and a television. Examples of those are shown in FIGS.15 and 16.

FIG. 15A is a portable telephone, and is composed of a main body 2601,an audio output portion 2602, an audio input portion 2603, a displayportion 2604, operation switches 2605, and an antenna 2606. The presentinvention can be applied to the display portion 2604.

FIG. 15B is a video camera, and is composed of a main body 2611, adisplay portion 2612, an audio input portion 2613, operation switches2614, a battery 2615 and an image receiving portion 2616. The presentinvention can be applied to the display portion 2612.

FIG. 15C is a mobile computer or a portable type information terminal,and is composed of a main body 2621, a camera portion 2622, an imagereceiving portion 2623, operation switches 2624, and a display portion2625. The present invention can be applied to the display portion 2625.

FIG. 15D is a head mount display, and is composed of a main body 2631, adisplay portion 2632, and an arm portion 2633. The present invention canbe applied to the display portion 2632.

FIG. 15E is a television, and is composed of a main body 2641, speakers2642, a display portion 2643, a receiving device 2644, and anamplification device 2645. The present invention can be applied to thedisplay portion 2643.

FIG. 15F is a portable electronic book, and is composed of a main body2651, a display device 2652, a memory medium 2653, an operation switch2654 and an antenna 2655. The book is used to display data stored in amini-disk (MD) or a DVD (Digital Versatile Disk), or a data receivedwith the antenna. The present invention can be applied to the displayportion 2652.

FIG. 16A is a personal computer, and is composed of a main body 2701, animage inputting portion 2702, a display device 2703 and a keyboard 2704.The present invention can be applied to the display portion 2703.

FIG. 16B is a player that employs a recording medium in which programsare recorded, and is composed of a main body 2711, a display portion2712, a speaker portion 2713, a recording medium 2714, and an operationswitch 2715. Note that this player uses a DVD (Digital Versatile Disc),CD and the like as the recording medium to appreciate music and films,play games, and connect to the Internet. The present invention can beapplied to the display portion 2612.

FIG. 16C is a digital camera comprising a main body 2721, a displayportion 2722, an eye piece 2723, operation switches 2724, and an imagereceiving portion (not shown). The present invention can be applied tothe display portion 2722.

FIG. 16D is a head mount display comprising a display portion 2731, anda band portion 2732. The present invention can be applied to the displayportion 2731.

Embodiment 8

FIG. 21 shows an example of applying the present invention to a portableinformation terminal. In this example, a video signal processing circuit2107 of a CPU 2106, VRAM 2111 and some other circuits stop theirfunctions during a still image is displayed to reduce power consumption.In FIG. 21, parts which operate are indicated by dotted lines. Acontroller 2112 may be mounted to a display device 2113 by COG orincorporated in the display device. FIGS. 22 and 23 show an example ofapplying the present invention to a cellular phone. Similar to the caseshown in FIG. 21, some functions can be stopped during still imagedisplay to reduce power consumption.

A plurality of memory circuits arranged in each pixel are used to storedigital video signals, so that the digital video signals stored in thememory circuits can be repeatedly used for every new frame period todisplay a still image continuously. Thus an external circuit, a sourcesignal line driver circuit, and other circuits can stop their operationsduring the still image is displayed. Furthermore, non-volatile memorycircuits arranged in each pixel are used to hold digital video signals,making it possible to keep the digital video signals stored after powersupply is shut off. Accordingly, the invention can greatly contribute tooverall power consumption reduction of a liquid crystal display device.

1. A display device comprising a pixel, the pixel comprising: a writinggate signal line; a reading gate signal line; a source signal line; acircuit able to keep digital data stored even after power supply is shutoff; a writing transistor, a gate of the writing transistor beingelectrically connected to the writing gate signal line, one of a sourceand a drain of the writing transistor being electrically connected tothe source signal line, and the other of the source and the drain of thewriting transistor being electrically connected to the circuit; and areading transistor, a gate of the reading transistor being electricallyconnected to the reading gate signal line, one of a source and a drainof the reading transistor being electrically connected to the circuit,and the other of the source and the drain of the reading transistorbeing electrically connected a pixel electrode.
 2. A display deviceaccording to claim 1, wherein the display device is able to display animage without newly sampling digital video signals, by reading digitaldata out of the circuit able to keep digital data even after powersupply is shut off.
 3. A display device according to claim 1, whereinthe display device is able to display an image without newly samplingdigital video signals even after the display is shut off and turned onagain.
 4. A display device according to claim 1, further comprising anon-volatile memory circuit in the circuit.
 5. A display deviceaccording to claim 1, further comprising a volatile memory circuit inthe circuit.
 6. A display device according to claim 1, furthercomprising a memory circuit selecting unit in the circuit.
 7. A displaydevice according to claim 6, wherein the memory circuit selecting unitcomprises a transistor
 8. A display device according to claim 1, furthercomprising an opposite electrode over the pixel electrode, a liquidcrystal material being interposed therebetween.
 9. A display deviceaccording to claim 1, further comprising a capacitor electricallyconnected to the reading transistor.
 10. A display device according toclaim 1, further comprising a driver circuit electrically connected tothe source signal line, wherein the driver circuit comprises a shiftregister, a latch circuit electrically connected to the shift registerand a video signal input line electrically connected to the latchcircuit.
 11. A display device according to claim 1, wherein the displaydevice is configured to store bits of a digital signal in the circuit.12. An electronic apparatus including the display device according toclaim 1, wherein the electronic apparatus is selected amongst atelevision set, a portable telephone, an electronic book, a personalcomputer, a portable terminal, a media player, a video camera, a digitalcamera and a head mounted display.
 13. A display device comprising apixel, the pixel comprising: writing gate signal lines; reading gatesignal lines; a source signal line; a circuit able to keep digital datastored even after power supply to the circuit is shut off; n×p writingtransistors, each of the writing transistors having a gate electricallyconnected to one of the writing gate signal lines, one of a source and adrain electrically connected to the source signal line, and the other ofthe source and the drain electrically connected to a correspondingmemory circuit; and n×p reading transistors, each of the readingtransistors having a gate electrically connected to one of the readinggate signal lines, one of a source and a drain electrically connected toa corresponding memory circuit, and the other of the source and thedrain electrically connected a pixel electrode, wherein n and p arenatural numbers.
 14. A display device according to claim 13, wherein thedisplay device is able to display an image without newly samplingdigital video signals, by reading digital data out of the circuit ableto keep digital data even after power supply is shut off.
 15. A displaydevice according to claim 13, wherein the display device is able todisplay an image without newly sampling digital video signals even afterthe display is shut off and turned on again.
 16. A display deviceaccording to claim 13, further comprising n×k non-volatile memorycircuit in the circuit, wherein k is a natural number verifying k≦p. 17.A display device according to claim 13, further comprising n×m volatilememory circuits in the circuit, wherein m is a natural number verifyingk≦p.
 18. A display device according to claim 13, further comprising amemory circuit selecting unit in the circuit.
 19. A display deviceaccording to claim 18, wherein the memory circuit selecting unitcomprises a transistor
 20. A display device according to claim 13,further comprising an opposite electrode over the pixel electrode, aliquid crystal material being interposed therebetween.
 21. A displaydevice according to claim 13, further comprising a capacitorelectrically connected to the reading transistors.
 22. A display deviceaccording to claim 13, further comprising a driver circuit electricallyconnected to the source signal line, wherein the driver circuitcomprises a shift register, a latch circuit electrically connected tothe shift register and a video signal input line electrically connectedto the latch circuit.
 23. A display device according to claim 13,wherein the display device is configured to store n bits of a digitalsignal in the circuit.
 24. A display device according to claim 16,wherein the display device is configured to store k frames of a digitalvideo signal in the circuit.
 25. A display device according to claim 17,wherein the display device is configured to store m frames of a digitalvideo signal in the circuit.
 26. An electronic apparatus including thedisplay device according to claim 13, wherein the electronic apparatusis selected amongst a television set, a portable telephone, anelectronic book, a personal computer, a portable terminal, a mediaplayer, a video camera, a digital camera and a head mounted display.